Diffusion bonded-superplastically formed line core hollow titanium fan blades are known in the prior art. The current design for a preform assembly for a wide chord fan blade (WCFB) comprises two opposing panels 2 sandwiching a membrane 3 onto which an internal core pattern is (e.g. silk-screen) printed—a general example of the preform assembly 1 is shown in FIG. 1(a), and a cross section of a final fan blade based on the preform of FIG. 1(a) is shown in FIG. 1(b), the cross-section being taken at the dashed line of FIG. 1(a) (but in the final fan blade rather than the preform assembly).
The two opposing panels 2 are typically first welded together e.g. at their edges, and the resulting assembly is diffusion bonded into a flat pack.
Then the assembly is heated through hot forming and an inert gas is injected between the two sheets, to inflate the assembly to form an internal cavity. The membrane, which is bonded to the titanium panels at the silk-printed locations, stretches by superplastic forming under the force exerted by the gas under a temperature greater than 900 degrees Celsius, typically generating a concertina type structure between the two panels in the internal cavity, as shown in FIG. 1(b) in cross section (taken at the dashed line in FIG. 1(a)).
Once the membrane has taken its final shape, the inert gas is removed; a vacuum being created between the two panels 2 to prevent any water contamination thanks to the pressure differential. The panels extend between a tip portion 4 and a root portion 5, for example as indicated in the preform assembly 1 in FIG. 1(a).
The superplastic membrane, which may take the form of an arrangement of prisms, forms the internal line-core of the blade and offers a strong resistance to stress and centrifugal force. An example of a cross-section of such a blade (post-production) is shown in FIG. 1(b) (looking from tip to root), showing the leading-edge 6 and the trailing edge 7 of the blade.